1. Field of the Invention
This invention relates to an automobile sensor for detecting the magnitude of a tensile force in a seat belt used in a car seat, and in particular to a sensor that can detect the magnitude of tension in a seat belt and provide an electrical signal that is representative of the magnitude of tensile force.
2. Description of the Related Art
Various devices are well known for their ability to measure force, pressure, acceleration, temperature, position, etc. by using a sensing structure combined with signal processing electronics. One general type of sensor or transducer for such applications is a resistive strain gauge sensor in which force or pressure is sensed or measured based on strain placed on the resistors. Resistive strain gauges function by exhibiting changes in resistance proportional to force which causes dimensional changes of the resistor.
Many types of strain gauge sensors have been designed and made commercially available. Various strain gauge sensors have proven to be generally satisfactory. Prior art sensors, however, have tended to be rather expensive and not suitable in certain applications such as sensing the presence of an occupant in an automobile seat. A sensor suitable for such an application must be compact, robust, impervious to shock and vibration and yet inexpensive.
Automobile seats can use sensors to activate air bags, which would be deployed during an accident. Injury to infants or small children from air bag deployment with excessive force is a current industry problem. A weight sensor in the seat can be used to control the deployment force during air bag activation.
Unfortunately there are several problems with detecting seat occupant weight. For example, when a seat occupant puts on a seat belt, the force of cinching down the seat belt on the occupant can cause a seat weight sensor to have false and erroneous readings. In another example, if a child's car seat is cinched down tightly in the car seat, it can appear to the weight sensor that a heavy person is in the seat, which is the wrong reading.
A current unmet need exists for a reliable, low cost, simple and robust seat belt tension sensor that can be used to input additional seat occupant information to an airbag controller to control airbag deployment.